IMAP stands for Internet Message Access Protocol and is one of the two most popular protocols for receiving email messages from the Internet, the other one being POP3. The main benefit of the IMAP protocol is that you can access your emails on more than one device because the content of your inbox will be checked directly on the server.
As an additional feature IMAP supports the usage of flags to define the state of the message - whether it has been read, replied to or deleted. Although these flags are stored on the server, note that different email clients handle them differently.